1. Field of the Invention
The invention is generally related to composite materials which include fiber reinforcements within a matrix material. More particularly, the invention is directed to improving the impact and perforation resistance of structures formed from composite materials.
2. Description of the Prior Art
Composite material laminates made from layers of carbon or graphite reinforcement fibers and a thermosetting polymer matrix generally have poor resistance to impact. Unlike metals which can deform plastically to dissipate impact energy, the stiff, highly-elastic composites generally lack a mechanism to dissipate energy beyond their yield or ultimate strength (see, Cantwell et al., Composites, 1991, Vol. 22, No. 5, pp. 347-362). The excess impact energy generates matrix cracks, ply delaminations, and, eventually, fiber breakage in the laminated composite materials and finally perforation. For thin laminate structures, the resulting damage (delaminations, cracks and fiber breakage) is often on the opposite side of the impact surface and, therefore, hidden from visual inspection (see, Cantwell et al., Composites Science and Technology, 1990, Vol. 38, pp. 119-141). For these and other reasons, impact damage resistance and damage tolerance are often limiting criteria when composites are considered for critical load bearing applications or impact puncture resistant armor applications.
A specific area of weakness in graphite reinforced composites is related to perforation resistance once damage has progressed in the composite beyond the delamination phase. However, few methods are known for reducing fiber fracture and material puncture once elastic strain energy storage capacity of the fibers has been exceeded.
Adding elastomeric compounds to the composite matrix ("rubber toughening"), interleaving thermoplastic layers into the composite laminate, using tougher reinforcing fibers, and hybridizing the composite laminate with tough aramid or polyethylene fibers are all methods that have been used to toughen graphite/epoxy composite laminates and increase perforation resistance. Various amounts of success have been attained using these methods; however, the results are not altogether satisfactory. Generally, the techniques work on the principle of increasing the capacity of the composite laminate to absorb or dissipate strain energy. Increasing the amount of strain energy that the composite laminate can dissipate elastically or inelastically before damage occurs reduces the amount of impact energy that remains to damage the laminate.
A shape memory alloy (SMA) is a metallic material which undergoes a transformation Go a martensite phase. In the martensite condition, the metallic material can be deformed in what appears to be a plastic manner; however, the material is actually deforming as a result of the growth and shrinkage of self-accommodating martensite plates. Recovery from the deformation results when the alloy is returned to its parent phase; hence, the name "shape memory alloy".
Shape memory alloys have been used in a wide variety of products including mechanical actuators, medical devices, and various control systems. Several examples of shape memory alloys are found in the patent literature. U.S. Pat. No. 4,717,341 to Goldberg discloses the use of the shape memory alloy, Nitinol, in orthodonic appliances. U.S. Pat. No. 4,909,510 to Sahatjian discloses a tennis racquet netting material made from a metal alloy which exhibits stress-induced martensite-martensite transformation of super elastic or psuedo elastic behavior, such as Nitinol or the like. U.S. Pat. No. 4,941,627 to Moscrip discloses the use of shape memory alloys in the fins of guided projectiles. U.S. Pat. No. 5,005,678 to Julien et al. discloses the use of shape memory alloys in an apparatus responsible for sensing and damping vibrations. U.S. Pat. No. 5,013,507 to Julien et al. discloses a method of producing a passage within a plastic material wherein a shape memory alloy is embedded into a molded plastic article, and, after hardening of the plastic, the shape memory alloy is pulled from the article to create a passage, whereby pulling the shape memory alloy causes it to elongate and assume a stress-induced matensitic state.
Shape memory effects can also be observed with other materials besides metal alloys. For example, U.S. Pat. No. 4,767,730 to Soma et al. discloses a ceramic shape memory element which employs zirconia within the matrix. U.S. Pat. No. 4,696,974 to Sulc et al. discloses a silicone composite that includes a powdered hydrophilic filler which has shape memory properties.